1. Field of the Invention
The invention relates to a semiconductor electronic device primarily used for high-power and/or high-frequency electric/electronic circuit. More specifically, the invention relates to e.g. diodes or transistors such as Schottky diodes, metal-semiconductor field effect transistors (MESFET), metal insulator semiconductor field effect transistors (MISFET), bipolar transistors, and heterobipolar transistors (HBT) using group III nitride semiconductor.
The invention also relates to a method of making such electronic devices.
The invention also relates to an epitaxial multi-layer wafer used to fabricate such electronic devices.
2. Description of the Existing Technology
(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III nitride alloys are the key semiconductor material for various electronic devices such as power switching transistors. Despite the fact that the maximum performance of GaN theoretically predicted with Baliga's Figure of Merit (BFOM) exceeds that of silicon carbide (SiC) by ˜5-fold, the lack of low-cost GaN wafers impedes development of GaN-based power switching transistors that can switch between two voltage levels quickly and with minimal loss. Currently, the majority of these devices are fabricated using a group III nitride film grown heteroepitaxially on a heterogeneous wafer, such as silicon, SiC and sapphire. However, heteroepitaxial growth of group III nitride results in highly defective or even cracked films. Typical defects in group III nitride heteroepitaxial films are threading dislocations at the level of 109 cm−2 along the growth direction. Because of this, vertical defects can become a current leakage path when high-voltage is applied vertically (i.e. along the growth direction). At this moment, GaN-based electronic devices are practically limited to horizontal devices such as high-electron mobility transistors (HEMT), which utilize current flow along the lateral directions near the surface. Since the electric current passes through a thin film in such horizontal devices, the thin film must have a large area to provide high-current (i.e. high-power) devices. In addition, all contacts are located on one side of the device, which makes the device much larger than a device having a vertical configuration. Due to these limitations, it is quite challenging to attain high-power devices in horizontal configuration of group III nitride semiconductors.
A homoepitaxial wafer or substrate such as GaN or AN is required to provide a GaN-based electronic device with a vertical configuration. The lack of low-cost and high crystallinity GaN substrates originates from difficulties in growing bulk crystal of GaN and other group III nitride compounds. Currently, the majority of commercially available GaN wafers are produced by hydride vapor phase epitaxy (HVPE). HVPE is a vapor phase method in which it is difficult to make GaN having a dislocation density less than 105 cm−2 when GaN grown on heteroepitaxial wafers (e.g. sapphire). Furthermore, the manufacturing process involves removal of the heteroepitaxial wafer after growing a thick (more than 0.1 mm) GaN layer, which is quite labor intensive and results in low yield.
Ammonothermal growth has been developed [1-6] to obtain low-cost, high crystallinity GaN substrates in which the density of dislocations and/or grain boundaries is less than 105 cm−2. The ammonothermal method is one of the bulk growth methods of group III nitride crystals using supercritical ammonia. Growth rate of crystals in supercritical ammonia is typically low. To grow bulk GaN crystals at a practically useful rate to produce substrates, a chemical additive called a mineralizer is added to the supercritical ammonia. A mineralizer is typically an element or a compound of group I elements or group VII elements, such as potassium, sodium, lithium, potassium amide, sodium amide, lithium amide, ammonium fluoride, ammonium chloride, ammonium bromide, ammonium iodide and gallium iodide. Sometimes more than two kinds of mineralizers are mixed to attain a good growth condition. Although most of the alkali-based mineralizers are interchangeable, sodium is the most favorable mineralizer in terms of growth rate, purity and handling. GaN substrates having dislocation density less than 105 cm−2 are produced using sodium mineralizer in ammonothermal growth. However, to achieve high-power electronic device having a vertical configuration with ammonothermal group III nitride substrates (in which one electrode is on one side or surface of a substrate and its corresponding electrode is formed on an opposite side or surface of the substrate so that the substrate resides between the two electrodes), an innovative device structure and fabrication method is required.